Sorry. I know this has been asked a hundred times but when i searched i could not find a correct answer as to what are the advantages / disadvantages that i could encounter if i switch my engine oil to Mobil1 5W40 or 5W50. I found that most reommended the 5W50 for the FIAT 1.6 engine but i am confused if it is ok to shift both the values (prior and after the 'W'). Why do most recommend 5W50 over 5W40 or Even 0W40 while moving to synthetic oil. That too when the FIAT recommemded grade is 15W40??

Hoping to get some good insights. ASAP since i am planning to change my oil by this weekend.

From what i understand those numbers stand for the viscosity of the oil when cold (value before the 'W') and hot (Value after 'W'). THis also means that higher the viscosity more time the oil takes to move through. In such case is it really advisable to move to a '50' rating oil when the company recommended one is only '40' (15W40).

Moving down from '15' to '5' logically does not cause any problems since it means that the oil would move around faster during cold starts. But my "laymans intellegence" tells me that the engine functions at very high temperatures and thus most of the time (except for initial few minutes) the engine is hot enough for the oil to be working at the higher viscosity rating. In that case i cannot understand why we would choose something that is more viscous than what is recommended thereby letting the oil take more time to move around??

In most parts in India the higher the right side number the better as it helps better against the heat so in extremely hot conditions 5-50 is better than 5-40 etc. The left side number is to protect against extreme cold. So IMHO unless the manufacture recommends it it doesn't really help going too low if it costs more. i.e is 10-40 is cheaper than 5-40 then may as well go with 10-40 if the manufacturer recommends 10 as the lower number.

Agree with gemithomas that moving to a lower left side number will allow btter flow of oil with lower viscosity, but I am not sure if there is any compromise in the oil formulation to allow a 0-50 range. I would prefer to go with 10-50 or even 15-50 in extremely hot areas.

In hot conditions like our summers the 5W50 will be better than 5W50.
However in hot summer 20W50 and 5W50 will perform similar.
If you are in sub zero, then 5W50 or 5W40 or 0W30 makes more sense than a 10W40 20W50 etc.,
You are in bangalore, and considering the ambient temperature 5W40 and 5W50 will give you similar results, rest everything being same.

In hot conditions like our summers the 5W50 will be better than 5W50.
However in hot summer 20W50 and 5W50 will perform similar.
If you are in sub zero, then 5W50 or 5W40 or 0W30 makes more sense than a 10W40 20W50 etc.,
You are in bangalore, and considering the ambient temperature 5W40 and 5W50 will give you similar results, rest everything being same.

thanks a lot. But waht bothers me is that the company recommends 15W40. I cant get 15W40 in synthetics.

I find most suggestions for the 5W50 synthetic.

My doubt is why is 5W50 recommended by most and not 5W40 (looking at company recommended values)

Sorry. I'm just confused and frustrated that i cant get a correct answere even after a lot of searching around.

I can see that U dont understand oil grades,gemi.
A 15W40 oil will work efficiently between 15 and 40(temperature unit,but neither centigrade or farenheit). A 5W50 will work efficiently between 5 and 50.
So, Now do u understand that the 5W50 will work like a 15W40 between 15 & 40?
Bottomline: The two numbers are basically a range.

nitrous has explained it.
So get an engine oil which has the rage equal to or higher than what company recommends.
But thats just the starting.
There is API rating also.
If your car says API-SL, make sure you get API-SL or higher(API-SM)